Cellular substrates of iron overload cardiomyopathies

Abstract

Cardiomyopathies and arrhythmias are major causes of death in untreated hereditary haemochromatosis, acute iron poisoning and during secondary iron overload resulting from repeated blood transfusions in β-thalassaemia. Iron overload cardiomyopathies are associated with systolic and diastolic dysfunction, suggesting that Ca2+ homeostasis is impaired. However, the cellular mechanisms of these dysfunctions are unknown. The data presented in this thesis establishes for the first time iron effects on cardiomyocyte Ca2+ handling, as well as the potential cellular substrates responsible for this impairment during iron overload. Exposure of isolated rat ventricular cardiomyocytes to 200μM iron led to biphasic changes in systolic Ca2+ release. Phase 1: an initial reduction of systolic Ca2+ release followed by; Phase 2: increased Ca2+ release with arrhythmogenic spontaneous Ca2+ release, cell contracture and cell death. There is evidence that Fe2+ enters cardiomyocytes via L-type Ca2+ channels (LTCC) and reduces the Ca2+ trigger. The close apposition of LTCCs to cardiac ryanodine receptors (RyR2) suggests RyR2 may be a first target. Indeed RyR2 activity was drastically reduced on exposure to nanomolar [Fe2+] in single channel studies. Together with evidence that Fe2+ may reduce the Ca2+ trigger from LTCC, this is consistent with iron reducing sarcoplasmic reticulum (SR) Ca2+ release during Phase 1. In Phase 2, the presence of spontaneous Ca2+ release events is consistent with SR Ca2+ overload. Indeed, in single rat ventricular cardiomyocytes SR Ca2+ content was found to be increased by 27% during Phase 2. The cellular substrates responsible for this increased SR Ca2+ content were 2-fold: 1) through reduced extrusion via both the Na+ Ca2+ Exchanger (NCX) and Plasmalemmal Ca2+ ATPase (PMCA) and 2) through increased resequestration via the SR Ca2+ ATPase. Iron catalyses the production of reactive oxygen species (ROS) during the Fenton reaction. To investigate whether iron effects might be due to ROS, I used the cell permeant ROS scavenger Tempol. Tempol attenuated Phase 2 effects but Phase 1 effects were not affected. This is consistent with the hypothesis that Phase 1 effects were due to direct effects of Fe2+ affecting LTCC trigger and RyR2 function. The attenuation of Phase 2 effects suggests that ROS damage to key Ca2+ handling mechanisms, such as NCX and PMCA might account for a reduced Ca2+ extrusion and subsequent SR Ca2+ overload.